Glycoproteins V and Ib-IX form a noncovalent complex in the platelet membrane.

Platelet glycoprotein (GP) V is a Mr 82,000 plasma membrane protein of unknown function that is cleaved by the potent platelet agonist, thrombin, to yield a Mr 69,500 fragment (GPVf1). Platelet GPIb, a disulfide-linked alpha beta heterodimer (Mr 160,000) that forms a noncovalent complex with GPIX (Mr 22,000), functions as the platelet adhesion receptor for surface-bound von Willebrand factor. Association between GPV and GPIb-IX has been suggested by the finding that both proteins are deficient in the Bernard-Soulier syndrome, a bleeding disorder characterized by giant platelets and defective interaction with von Willebrand factor. Here we report that GPV and GPIb-IX are coprecipitated by monoclonal antibodies (mAbs) against GPV, GPIb, or GPIX when platelets are solubilized in the mild detergent, digitonin. Treatment of digitonin immunopreciptates with the nonionic detergent, Nonidet P-40, released GPV from anti-GPIb and anti-GPIX mAb precipitates and GPIb-IX from the anti-GPV mAb precipitate. Removal of the Mr 45,000 amino-terminal part of GPIb alpha by treatment with elastase did not abrogate association of GPV with GPIb-IX, showing that the leucine-rich repeat sequences in GPIb alpha are not required for complex formation. Binding studies with 125I-labeled mAbs showed the presence of 24,370 GPIb-IX complexes and 11,170 molecules of GPV/platelet (n = 5). These data show that the leucine-rich glycoproteins GPV and GPIb-IX form a noncovalent complex in the platelet membrane. GPV may play a role in the interaction of platelets with von Willebrand factor.     

Regulation of glycoprotein Ib-IX-von Willebrand factor interaction by cAMP-dependent protein kinase-mediated phosphorylation at Ser 166 of glycoprotein Ib(beta)

The platelet receptor for von Willebrand factor (VWF), glycoprotein (GP) Ib-IX, mediates initial platelet adhesion and activation. It is known that the cytoplasmic domain of GPIbbeta is phosphorylated at Ser(166) by cAMP-dependent protein kinase (PKA). To understand the physiological role of GPIbbeta phosphorylation, a GPIb-IX mutant replacing Ser(166) of GPIbbeta with alanine (S166A) and a deletion mutant lacking residues 166-181 of GPIbbeta (Delta165) were constructed. These mutants, expressed in Chinese hamster ovary (CHO) cells, showed an enhanced VWF-binding function compared with wild type GPIb-IX. Treatment of CHO cells expressing wild type GPIb-IX with a PKA inhibitor, PKI, reduced Ser(166) phosphorylation and also enhanced VWF binding to GPIb-IX. Furthermore, cells expressing S166A or Delta165 mutants showed a significantly enhanced adhesion to immobilized VWF under flow conditions. Consistent with the studies in CHO cells, treatment of platelets with PKI enhanced VWF binding to platelets. In contrast, a PKA stimulator, forskolin, reduced VWF binding and VWF-induced platelet agglutination, which was reversed by PKI. Thus, PKA-mediated phosphorylation of GPIbbeta at Ser(166) negatively regulates VWF binding to GPIb-IX and is one of the mechanisms by which PKA mediates platelet inhibition.     

Binding of anticoagulant vitamin K-dependent protein S to platelet-derived microparticles.

Vitamin K-dependent protein S is an anticoagulant plasma protein serving as cofactor to activated protein C in degradation of coagulation factors Va and VIIIa on membrane surfaces. In addition, it forms a noncovalent complex with complement regulatory protein C4b-binding protein (C4BP), a reaction which inhibits its anticoagulant function. Both forms of protein S have affinity for negatively charged phospholipids, and the purpose of the present study was to elucidate whether they bind to the surface of activated platelets or to platelet-derived microparticles. Binding of protein S to human platelets stimulated with various agonists was examined with FITC-labeled monoclonal antibodies and fluorescence-gated flow cytometry. Protein S was found to bind to membrane microparticles which formed during platelet activation but not to the remnant activated platelets. Binding to microparticles was saturable and maximum binding was seen at approximately 0.4 microM protein S. It was calcium-dependent and reversed after the addition of EDTA. Inhibition experiments with monoclonal antibodies suggested the gamma-carboxyglutamic acid containing module of protein S to be involved in the binding reaction. An intact thrombin-sensitive region of protein S was not required for binding. The protein S-C4BP complex did not bind to microparticles or activated platelets even though it bound to negatively charged phospholipid vesicles. Intact protein S supported binding of both protein C and activated protein C to microparticles. Protein S-dependent binding of protein C/activated protein C was blocked by those monoclonal antibodies against protein S that inhibited its cofactor function. In conclusion, we have found that free protein S binds to platelet-derived microparticles and stimulates binding of protein C/activated protein C.(ABSTRACT TRUNCATED AT 250 WORDS)     

Platelet glycoprotein Ib: a zinc-dependent binding protein for the heavy chain of high-molecular-weight kininogen.

Domains 3 and 5 of high-molecular-weight kininogen (HK) have been shown to bind to platelets in a zinc-dependent reaction. However, the platelet-binding proteins responsible for this interaction have not been identified. We have focused on the platelet-binding site for the heavy chain (domain 3), which we approached using a domain 3-derived peptide ligand and isolated binding proteins by affinity chromatography. The domain 3-derived peptide, thrombin, HK, factor XII, as well as antibody to glycocalicin (the N-terminal portion of the alpha chain of GPIb) recognized a protein at 74 kD. We also isolated the thrombin receptor (PAR 1) at 45 kD, however, none of the above-mentioned ligands bound to this protein. Isolation of platelet membrane proteins using a monoclonal anti-glycocalicin antibody column revealed the same HK binding protein at 74 kD, which was reactive with anti-GPIb and represents a GPIb fragment. By photoaffinity labeling, HK interacted with membrane GPIb, which was then isolated in native form (135 kD) along with gC1qR, a ligand for the HK light chain. Finally, (125)I-HK binding to platelets was significantly inhibited by the anti-GPIb antibody. These results suggest that the GPIb alpha chain, a known thrombin binding protein, is also one of the zinc-dependent platelet membrane binding sites for HK domain 3.     

Regulation of von Willebrand factor binding to the platelet glycoprotein Ib-IX by a membrane skeleton-dependent inside-out signal

The platelet receptor for von Willebrand factor (vWF), glycoprotein Ib-IX (GPIb-IX), mediates initial platelet adhesion and activation. We show here that the receptor function of GPIb-IX is regulated intracellularly via its link to the filamin-associated membrane skeleton. Deletion of the filamin binding site in GPIb(alpha) markedly enhances ristocetin- (or botrocetin)-induced vWF binding and allows GPIb-IX-expressing cells to adhere to immobilized vWF under both static and flow conditions. Cytochalasin D (CD) that depolymerizes actin also enhances vWF binding to wild type GPIb-IX. Thus, vWF binding to GPIb-IX is negatively regulated by the filamin-associated membrane skeleton. In contrast to native vWF, binding of the isolated recombinant vWF A1 domain to wild type and filamin binding-deficient mutants of GPIb-IX is comparable, suggesting that the membrane skeleton-associated GPIb-IX is in a state that prevents access to the A1 domain in macromolecular vWF. In platelets, there is a balance of membrane skeleton-associated and free forms of GPIb-IX. Treatment of platelets with CD increases the free form and enhances vWF binding. CD also reverses the inhibitory effects of prostaglandin E1 on vWF binding to GPIb-IX. Thus, GPIb-IX-dependent platelet adhesion is doubly controlled by vWF conformation and a membrane skeleton-dependent inside-out signal.     

Reconstitution of the purified platelet fibrinogen receptor. Fibrinogen binding properties of the glycoprotein IIb-IIIa complex

Several lines of evidence indicate that the platelet membrane glycoprotein IIb-IIIa complex (GP IIb-IIIa) is necessary for the expression of platelet fibrinogen receptors. The purpose of the present study was to determine whether purified GP IIb-IIIa retains the properties of the fibrinogen receptor on platelets. Glycoprotein IIb-IIIa was incorporated by detergent dialysis into phospholipid vesicles composed of 30% phosphatidylcholine and 70% phosphatidylserine. 125I-Fibrinogen binding to the GP IIb-IIIa vesicles, as measured by filtration, had many of the characteristics of 125I-fibrinogen binding to whole platelets or isolated platelet plasma membranes: binding was specific, saturable, reversible, time dependent, and Ca2+ dependent. The apparent dissociation constant for 125I-fibrinogen binding to GP IIb-IIIa vesicles was 15 nM, and the maximal binding capacity was 0.1 mol of 125I-fibrinogen/mol of GP IIb-IIIa. 125I-Fibrinogen binding was inhibited by amino sugars, the GP IIb and/or IIIa monoclonal antibody 10E5, and the decapeptide from the carboxyl terminus of the fibrinogen gamma chain. Furthermore, little or no 125I-fibrinogen bound to phospholipid vesicles lacking protein or containing proteins other than GP IIb-IIIa (i.e. bacteriorhodopsin, apolipoprotein A-I, or glycophorin). Also, other 125I-labeled plasma proteins (transferrin, orosomucoid) did not bind to the GP IIb-IIIa vesicles. These results demonstrate that GP IIb-IIIa contains the platelet fibrinogen receptor.     

Glycoprotein (GP) Ib-IX-transfected cells roll on a von Willebrand factor matrix under flow. Importance of the GPib/actin-binding protein (ABP-280) interaction in maintaining adhesion under high shear

Adhesion of platelets to sites of vascular injury is critical for hemostasis and thrombosis and is dependent on the binding of the vascular adhesive protein von Willebrand factor (vWf) to the glycoprotein (GP) Ib-V-IX complex on the platelet surface. A unique but poorly defined characteristic of this receptor/ligand interaction is its ability to support platelet adhesion under conditions of high shear stress. To examine the structural domains of the GPIb-V-IX complex involved in mediating cell adhesion under flow, we have expressed partial (GPIb-IX), complete (GPIb-V-IX), and mutant (GPIbalpha cytoplasmic tail mutants) receptor complexes on the surface of Chinese hamster ovary (CHO) cells and examined their ability to adhere to a vWf matrix in flow-based adhesion assays. Our studies demonstrate that the partial receptor complex (GPIb-IX) supports CHO cell tethering and rolling on a bovine or human vWf matrix under flow. The adhesion was specifically inhibited by an anti-GPIbalpha blocking antibody (AK2) and was not observed with CHO cells expressing GPIbbeta and GPIX alone. The velocity of rolling was dependent on the level of shear stress, receptor density, and matrix concentration and was not altered by the presence of GPV. In contrast to selectins, which mediate cell rolling under conditions of low shear (20-200 s-1), GPIb-IX was able to support cell rolling at both venous (150 s-1) and arterial (1500-10,500 s-1) shear rates. Studies with a mutant GPIbalpha receptor subunit lacking the binding domain for actin-binding protein demonstrated that the association of the receptor complex with the membrane skeleton is not essential for cell tethering or rolling under low shear conditions, but is critical for maintaining adhesion at high shear rates (3000-6000 s-1). These studies demonstrate that the GPIb-IX complex is sufficient to mediate cell rolling on a vWf matrix at both venous and arterial levels of shear independent of other platelet adhesion receptors. Furthermore, our results suggest that the association between GPIbalpha and actin-binding protein plays an important role in enabling cells to remain tethered to a vWf matrix under conditions of high shear stress.     

Purification and identification of the lipoprotein-binding proteins from human blood platelet membrane

As reported previously, homologous plasma lipoproteins specifically bind to the plasma membrane of human blood platelets. The two major lipoprotein-binding membrane glycoproteins were purified to apparent homogeneity and identified by their mobilities in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, both in the nonreduced and reduced state, by specific antibodies against glycoproteins IIb (GPIIb) and IIIa (GPIIIa), respectively, including the alloantibody anti-PlA1 and monoclonal antibodies. Furthermore, lipoprotein binding to intact platelets is also inhibited in a dose-dependent fashion by preincubation of the platelets with antibodies against these glycoproteins. From these experiments it can be concluded that lipoproteins bind to both components of the glycoprotein IIb-IIIa complex in isolated membranes and intact platelets. High density lipoprotein and low density lipoprotein bind to GPIIIa blotted to nitrocellulose in a way that binding of one species interferes with the binding of the other. Addition of fibrinogen significantly inhibits this binding. The specific binding of fibrinogen to GPIIIa is strongly inhibited in the presence of either of the two lipoproteins. LDL and HDL are specifically bound by isolated GPIIb, too. In our blotting experiments fibrinogen shows no binding to this membrane glycoprotein. On the other hand, fibrinogen significantly interferes with the interaction between GPIIb and the lipoproteins.     

Activation affects access to the platelet receptor for adhesive glycoproteins

Blood platelets have a receptor for macromolecular adhesive glycoproteins, located on a heteroduplex membrane glycoprotein complex (GPIIb/IIIa) that only becomes "exposed" when platelets are activated. Binding of the adhesive glycoproteins, in particular fibrinogen, to the receptor is required for platelet aggregation, which in turn is required to arrest bleeding. A murine monoclonal antibody whose rate of binding to the receptor is affected by platelet activation was both cross-linked and fragmented to assess the effects of changes in molecular size on its rate of binding to unactivated and activated platelets. The results indicate that small molecules can bind more rapidly to the receptors on unactivated platelets than can large molecules and that activation involves a conformational and/or microenvironmental change that permits the large molecules to bind more rapidly.     

Ristocetin-dependent reconstitution of binding of von Willebrand factor to purified human platelet membrane glycoprotein Ib-IX complex

Whether the human platelet membrane glycoprotein (GP) Ib-IX complex is the receptor for ristocetin-dependent binding of von Willebrand factor (vWF) has been examined by reconstitution with the purified components using a solid-phase bead assay. Purified GP Ib-IX complex was bound and orientated on the beads via a monoclonal antibody, FMC 25, directed against the membrane-associated region of the complex. Specific binding of 125I-labeled vWF to the GP Ib-IX complex coated beads was strictly ristocetin dependent with maximal binding occurring at ristocetin concentrations greater than or equal to 1 mg/mL. Ristocetin-dependent specific binding of 125I-labeled vWF was saturable. The observed binding was specific to the interaction between vWF and the GP Ib-IX complex since there was no ristocetin-dependent specific binding of vWF if the physicochemically related platelet membrane glycoprotein, GP IIb, was substituted for the GP Ib-IX complex in a corresponding bead assay. Further, neither bovine serum albumin nor other adhesive glycoproteins, such as fibrinogen or fibronectin, specifically bound to the GP Ib-IX complex in the presence of ristocetin. Ristocetin-dependent binding of vWF to platelets and to GP Ib-IX complex coated beads was inhibited by monoclonal antibodies against a 45,000 molecular weight N-terminal region of GP Ib but not by monoclonal antibodies directed against other regions of the GP Ib-IX complex. Similar correspondence between platelets and purified GP Ib-IX complex with respect to the ristocetin-dependent binding of vWF was obtained with anti-vWF monoclonal antibodies.(ABSTRACT TRUNCATED AT 250 WORDS)     

Identification of two structurally and functionally distinct sites on human platelet membrane glycoprotein IIb-IIIa using monoclonal antibodies

Platelet membrane glycoprotein IIb-IIIa exists as a calcium-dependent complex of two large peptides (designated IIb and IIIa) in Triton X-100 solutions, but it remains unknown if these peptides are subunits of one glycoprotein or are actually two individual glycoproteins in the intact platelet membrane. We used crossed immunoelectrophoresis to define the epitopes of two monoclonal antibodies to IIb-IIIa, then used these antibodies to study the structural and functional organization of IIb and IIIa in the platelet membrane. Human platelets solubilized in Triton X-100 were electrophoresed through an intermediate gel containing 125I-monoclonal IgG, then into an upper gel containing rabbit anti-human platelet antibodies. Our previously characterized antibody. Tab, and a new monoclonal antibody, T10, both bound to the immunoprecipitate corresponding to the IIb-IIIa complex. When platelets were electrophoresed after solubilization in 5 mM EDTA, 125I-Tab bound to the dissociated IIb polypeptide, but not to IIIa. In contrast, 125-I-T10 did not react with either IIb or IIIa. Thus, Tab recognizes a determinant on IIb, while T10 recognizes a determinant created only after the association of IIb and IIIa. Gel-filtered platelets from six normal donors bound 50,600 +/- 5,600 125I-T10 molecules/platelet and 47,800 +/- 11,200 125I-Tab molecules/platelet, consistent with IIb-IIIa being a heterodimer. 125I-T10 binding was identical in unactivated platelets and platelets stimulated with 10 microM ADP. However, platelets did not aggregate or bind 125I-fibrinogen until ADP was added. T10, but not Tab or nonimmune mouse antibody, inhibited ADP-induced platelet aggregation and 125I-fibrinogen binding. Our findings suggest that IIb and IIIa exist as subunits of a single membrane glycoprotein in unstimulated platelets. Fibrinogen binding appears to require not only the interaction of IIb and IIIa, but also some additional change occurring after platelet activation.     

Functional reconstitution of Beta2-adrenergic receptors utilizing self-assembling Nanodisc technology

Integral membrane G protein-coupled receptors (GPCRs) compose the single most prolific class of drug targets, yet significant functional and structural questions remain unanswered for this superfamily. A primary reason for this gap in understanding arises from the difficulty of forming soluble, monodisperse receptor membrane preparations that maintain the transmembrane signaling activity of the receptor and provide robust biophysical and biochemical assay systems. Here we report a technique for self-assembling functional beta2-adrenergic receptor (beta2AR) into a nanoscale phospholipid bilayer system (Nanodisc) that is highly soluble in aqueous solution. The approximately 10-nm nanobilayer particles contain beta2AR in a native-like phospholipid bilayer domain of approximately 100 phospholipid molecules circumferentially bound by a membrane scaffold protein (MSP). The resulting construct allows for access to the physiologically intracellular and extracellular faces of the receptor and thus allows unrestricted access of antagonists, agonists, and G proteins. These Nanodisc-solubilized GPCRs can be directly purified by normal chromatographic procedures. We define the resultant Nanodisc-embedded monomeric beta2AR by antagonist and agonist binding isotherms and demonstrate faithful G protein coupling.     

PADGEM protein: a receptor that mediates the interaction of activated platelets with neutrophils and monocytes

PADGEM (platelet activation dependent granule-external membrane protein) is an integral membrane protein of the alpha granules of platelets and Weibel-Palade bodies of endothelial cells that is expressed on the plasma membrane upon cell activation and granule secretion. Activated platelets, but not resting platelets, bind to neutrophils, monocytes, HL60 cells, and U937 cells. This interaction is inhibited by anti-PADGEM antibodies, PADGEM, and EDTA; anti-GPIIb-IIIa, anti-thrombospondin, anti-GPIV, and thrombospondin produce no effect. Neutrophils and U937 cells, in contrast to Jurkatt cells, contain PADGEM recognition sites, as shown by binding of PADGEM contained in phospholipid vesicles. These results indicate that PADGEM mediates adhesion of activated platelets to monocytes and neutrophils. Therefore, PADGEM shares not only structural but also functional homology with ELAM-1 and MEL-14, members of a new family of vascular cell adhesion molecules.     

Shear stress-induced binding of von willebrand factor to platelets

Shear stress-induced platelet aggregation requires von Willebrand factor (vWF), platelet glycoprotein (GP) Ib, GPIIb-IIIa, Ca²⁺, and adenosine diphosphate (ADP). Recent reports using vWF labeled with either ¹²⁵I or fluorescein isothiocyanate (FITC) have demonstrated that in shear-fields, vWF binds to both GPIb and GPHb-IIIa. The sequence of the vWF binding to the two platelet receptors has not been precisely determined in these reports. In this study, a flow cytometry technique using a primary anti-vWF antibody and a secondary FITC IgG antibody was used to measure shear stress-induced vWF binding to platelets. Washed normal platelets suspended at 50,000/μl with purified large VWF multimers were exposed to laminar shear stresses of 15 to 120 dynes/cm² for 30 sec. At this low platelet count, little or no aggregation occurred in the shear fields. A significant increase in post-shear vWF-positive platelets was consistently observed. Experiments with platelets from normal and severe von Willebrand's disease (vWD) (which lack plasma and platelet α-granule vWF) demonstrated that exogenous vWF predominately contributed to the platelet-vWF binding. Blockade of platelet GPIb with the monoclonal anti-GPIb antibody, 6D1, completely inhibited shear stress-induced platelet-vWF attachment. In contrast, blockade of GPIIb-IIIa with monoclonal anti-GPIIb-IIIa antibodies, 10E5 or c7E3, or with the GPIIb-IIIa-blocking tetrapeptide, RGDS, had little or no inhibitory effect on platelet-vWF binding. These data demonstrate that the binding of vWF to GPIb is likely to be the initial shear-induced platelet-ligand binding event. © 1997 Elsevier Science Ltd     

A 90,000-dalton binding protein common to both steroid receptors and the Rous sarcoma virus transforming protein, pp60v-src

Previous studies have shown that the avian progesterone receptor, when in the nontransformed 8 S state, is complexed to another cellular protein having a molecular weight of 90,000. In this report, we show that this receptor-binding protein is indistinguishable from the 90,000-dalton protein which associates in a complex with the Rous sarcoma virus transforming protein, pp60v-src. This identity was established by the following criteria. 1) Monoclonal antibodies directed against the pp60v-src-associated 90-kDa protein recognized the 90-kDa progesterone receptor binding protein in an immunoblot assay. Conversely, monoclonal antibodies that recognize the progesterone receptor binding protein bind to the 90-kDa protein which complexes with pp60v-src. 2) Peptide maps prepared from the 90-kDa proteins immunoprecipitated from chicken cells with monoclonal antibodies directed against either the 90-kDa receptor binding protein or the 90-kDa pp60v-src-associated protein were indistinguishable. 3) Preincubation of the progesterone receptor complex with monoclonal antibodies prepared against the pp60v-src-associated protein caused a shift in the sedimentation of the progesterone receptor. Previous studies have established that the pp60v-src-associated protein is indistinguishable from one of the major heat shock proteins which are induced under a variety of stress conditions in eukaryotic cells. These present studies implicate a new role for this 90-kDa protein in the action of steroid hormones.     

Membrane protein assembly into Nanodiscs

Nanodiscs are soluble nanoscale phospholipid bilayers which can self-assemble integral membrane proteins for biophysical, enzymatic or structural investigations. This means for rendering membrane proteins soluble at the single molecule level offers advantages over liposomes or detergent micelles in terms of size, stability, ability to add genetically modifiable features to the Nanodisc structure and ready access to both sides of the phospholipid bilayer domain. Thus the Nanodisc system provides a novel platform for understanding membrane protein function. We provide an overview of the Nanodisc approach and document through several examples many of the applications to the study of the structure and function of integral membrane proteins.     

Binding of annexin V/placental anticoagulant protein I to platelets. Evidence for phosphatidylserine exposure in the procoagulant response of activated platelets

Proteins of the annexin/lipocortin family act as in vitro anticoagulants by binding to anionic phospholipid vesicles. In this study, we investigated whether annexin V (placental anticoagulant protein I) would bind to human platelets. Annexin V bound to unstimulated platelets in a reversible, calcium-dependent reaction with an apparent Kd of 7 nM and 5000-8000 sites/platelet. Additional binding sites could be induced by several platelet agonists in the following order of effectiveness: A23187 greater than collagen + thrombin greater than collagen greater than thrombin. However, neither ADP nor epinephrine induced additional binding sites. Three other proteins of the annexin family (annexins II, III, and IV) competed for annexin V platelets binding sites with the same relative potencies previously observed for binding to phospholipid vesicles. Phospholipid vesicles containing phosphatidylserine completely inhibited binding of annexin V to platelets. Annexin V completely blocked binding of 125I-factor Xa to thrombin-stimulated platelets. These results support the hypothesis that phosphatidylserine exposure occurs during platelet activation and may be necessary for assembly of the prothrombinase complex on platelet membranes.     

Red cell ICAM-4 is a novel ligand for platelet-activated alpha IIbbeta 3 integrin

ICAM-4 (LW blood group glycoprotein) is an erythroid-specific membrane component that belongs to the family of intercellular adhesion molecules and interacts in vitro with different members of the integrin family, suggesting a potential role in adhesion or cell interaction events, including hemostasis and thrombosis. To evaluate the capacity of ICAM-4 to interact with platelets, we have immobilized red blood cells (RBCs), platelets, and ICAM-Fc fusion proteins to a plastic surface and analyzed their interaction in cell adhesion assays with RBCs and platelets from normal individuals and patients, as well as with cell transfectants expressing the alpha(IIb)beta(3) integrin. The platelet fibrinogen receptor alpha(IIb)beta(3) (platelet GPIIb-IIIa) in a high affinity state following GRGDSP peptide activation was identified for the first time as the receptor for RBC ICAM-4. The specificity of the interaction was demonstrated by showing that: (i) activated platelets adhered less efficiently to immobilized ICAM-4-negative than to ICAM-4-positive RBCs, (ii) monoclonal antibodies specific for the beta(3)-chain alone and for a complex-specific epitope of the alpha(IIb)beta(3) integrin, and specific for ICAM-4 to a lesser extent, inhibited platelet adhesion, whereas monoclonal antibodies to GPIb, CD36, and CD47 did not, (iii) activated platelets from two unrelated type-I glanzmann's thrombasthenia patients did not bind to coated ICAM-4. Further support to RBC-platelet interaction was provided by showing that dithiothreitol-activated alpha(IIb)beta(3)-Chinese hamster ovary transfectants strongly adhere to coated ICAM-4-Fc protein but not to ICAM-1-Fc and was inhibitable by specific antibodies. Deletion of individual Ig domains of ICAM-4 and inhibition by synthetic peptides showed that the alpha(IIb)beta(3) integrin binding site encompassed the first and second Ig domains and that the G65-V74 sequence of domain D1 might play a role in this interaction. Although normal RBCs are considered passively entrapped in fibrin polymers during thrombus, these studies identify ICAM-4 as the first RBC protein ligand of platelets that may have relevant physiological significance.     

von Willebrand factor. A protein which binds at the cell surface interface between platelets

von Willebrand factor (VWF) functions in platelet aggregation, a form of cellular interaction. In vitro analysis of platelet aggregation, as measured by the platelet aggregometer, requires addition of a promoter such as the glycopeptide antibiotic ristocetin. Native multimeric VWF (Mr = 1-20 X 10(6)) can be reduced with sulfhydryl reagents to a monomeric state (Mr = 2 X 10(5)). In this study, the binding of bovine VWF and ristocetin to bovine platelets was investigated using fluorescence anisotropy of derivatized monomer protein and ristocetin and also by radioisotope methods using 125I-labeled monomer and native protein. Ristocetin bound to bovine platelets but not to VWF. VWF binding to formaldehyde-fixed platelets was dependent on the presence of a promoter such as ristocetin. The monomer and multimer VWF bound equally well in the presence of low ristocetin concentrations. Under these conditions, plots of VWF binding versus platelet concentration were sigmoidal, indicating positive cooperativity with respect to platelets. At higher (100 micrograms/ml) ristocetin concentrations, the binding curve was no longer sigmoidal. Ristocetin promoted the formation of small platelet aggregates, an effect that was amplified by the presence of VWF. In fact, all conditions which resulted in monomer or multimer VWF binding to platelets also caused formation of platelet aggregates observed by light microscopy. These combined results were consistent with VWF binding only to the interface between proximal platelets. High affinity binding could be provided by the presence of two cell surfaces and the resulting multiple binding interactions. Polycations, such as poly(L-lysine) and Polybrene, also promoted the formation of platelet aggregates and facilitated the binding of VWF to platelets. Physiological platelet activators such as thrombin, ADP, and collagen also facilitated VWF binding to native platelets and caused platelet aggregation. It appears possible that any process which causes the surface membranes of platelets to become spatially close will allow expression of VWF activity.